Phase change material containing fusible particles as thermally conductive filler

ABSTRACT

An electronic assembly having a microelectronic die, a heat spreader and a heat sink. A first thermal interface material is disposed between the microelectronic die and the heat spreader. A second thermal interface material is disposed between the heat spreader and a heat sink. The first and second interface materials each comprising a phase change polymer, a solderable material and a plurality of thermally conductive non-fusible particles. The solderable material interconnecting the non-fusible particles to form a plurality of columnar structures within the phase change polymer.

This is a Divisional of application Ser. No. 11/129,516 filed May 13,2005 now U.S. Pat. No. 7,294,394, which is a Divisional of applicationSer. No. 10/071,743 filed Feb. 8, 2002 now U.S. Pat. No. 6,926,955issued Aug. 9, 2005.

FIELD OF THE INVENTION

The present invention relates to the field of computer componentassembly and in particular to a thermal interface material placedbetween computer components at assembly.

BACKGROUND OF THE INVENTION

The need for smaller and faster computer chips has caused a dramaticincrease in the power needed to remove from the chip. This is made moredifficult by the shrinking of the die and the larger heat flux per unitarea. Thermal interface materials (TIMs) have a key function in a flipchip package, i.e. to dissipate heat to allow higher processing speeds.More specifically, thermal interface materials bring the die into goodthermal contact with the heat removal hardware.

Thermal interface materials are available in a wide variety of formulasfrom silicone and non-silicone bases filled with metal oxides. The metaloxide particles provide the high thermal conductivity to the compound.The ability to fill the tiny cavities of mating surfaces will depend onthe metal oxide particle sizes. The particles are designed to give thehighest thermal conductivity to the compound. The lowest thermalresistance is a combination of high thermal conductivity and the abilityof the material to penetrate all of the cavities and fill all the spacescreated by any non-flat areas of the two mating surfaces. Thermal greaseprovides the lowest thermal resistance interface available (notincluding a soldered type connection). The disadvantage of thermalgrease is the inconsistency of application and the problem of keeping itfrom being messy to use. There are many grease application productsavailable today to help with the ease of use and keeping it where itbelongs, such as spraying, screening, sticks and pads (pads are a greasethat is dry to the touch).

Attaching a heat sink to a semiconductor package requires that two solidsurfaces be brought together into intimate contact. Unfortunately, nomatter how well prepared, solid surfaces are never really flat or smoothenough to permit intimate contact. All surfaces have a certain roughnessdue to microscopic hills and valleys. Superimposed on this surfaceroughness is a macroscopic non-planarity in the form of a concave,convex or twisted shape. As two such surfaces are brought together, onlythe hills of the surfaces come into physical contact. The valleys areseparated and form air-filled gaps. When two typical electroniccomponent surfaces are brought together, less than one percent of thesurfaces may make physical contact with the remainder (99%) of thesurfaces separated by a layer of interstitial air. Some heat isconducted through the physical contact points, but much more has totransfer through the air gaps. Since air is a poor conductor of heat, itshould be replaced by a more conductive material to increase the jointconductivity and thus improve heat flow across the thermal interface.

Several types of thermally conductive materials can be used as TIMs toeliminate air gaps from a thermal interface including greases, reactivecompounds, elastomers, and pressure sensitive adhesive films. All aredesigned to conform to surface irregularities, thereby eliminating airvoids and improving heat flow through the thermal interface.

A TIM can be made from a polymer matrix and a highly thermallyconductive filler. TIMs find three application areas in a CPUpackage: 1) to bring a bare die package into contact with a heatspreader (FIG. 1A), 2) to bring the die into good thermal contact withan integrated heat sink hardware (FIG. 1B), and 3) to bring the heatspreader into contact with OEM applied hardware (FIG. 1B). The TIMplaced between the die or die package and heat spreader is called a TIM1 and the TIM placed between the heat spreader and heat sink hardware isreferred to as a TIM 2.

Historically, soft polymers used in TIMs have been silicones, epoxies,urethanes, acrylates and olefins. Filler types have ranged dramaticallyfrom inexpensive aluminum oxides and zinc oxide to aluminum, boronnitride, silver, graphite, carbon fibers, and diamond. Phase change TIMsare a class of polymer materials that undergo a transition from a solidto a liquid phase with the application of heat. The phase change TIMsare a soft solid at room temperature but a thick fluid at operatingtemperature. This transition occurs due to the presence of a low meltingsolid, typically a wax, mixed with the polymer in the presence of highlyconductive filler. Due to the transition, phase change materials readilyconform to surfaces and provide low thermal resistance and higher heatremoval capability.

A heatpipe is a heat transfer or heat sink structure that can include anumber of channels for transferring heat from one end to a condenserregion at the other end. Each heatpipe can be composed of a centralvapor channel with a number of parallel capillary channels (not shown),each of which is open on one side to the vapor channel thereby servingas the wick of the heat pipe, running the length of the heatpipe to acondenser region. The heat from the microchip vaporizes a working fluidin the capillaries and the vapor in turn travels in the vapor channel tothe condenser region to be cooled and condensed by a cooling medium,such as air, present over this region.

When a heatpipe is used, a heatpipe surface contacting the circuitpackage can have a cross-section smaller than the circuit package itcontacts and a portion of the circuit package may extend out beyond theheatpipe edges. As a result, heat transfer may not be as efficient asrequired and a thermal adaptor such as a spreader plate may be used as aheat spreader to compensate. To improve thermal conduction between theheatpipe and the circuit package, the spreader plate can have a surfacearea and shape that more closely matches with the circuit package whenthe spreader plate is positioned between the heatpipe and the circuitpackage.

FIG. 2 is an illustration of an arrangement of a non-fusible particlefiller material within the polymer matrix of a TIM. The non-fusibleparticles, such as metals, benefit from a high thermal conductivity,however, a thermal flow path through the TIM is limited by thepoint-to-point contact of the particles as shown by the arrows. Withinthe TIM, these particles being non-fusible (i.e. will not melt and flowduring normal processing and so remain as point contacts with eachother) result in thermal conductivity through the TIM that is amechanism referred to as percolation. The phenomenon of percolationdescribes the effects of interconnections present in a random system,here the number of filler particles that are randomly in point contactwith each other to allow thermal conduction. Normally, to improveconduction limited by percolation, the amount of filler could beincreased until a threshold amount is reached and heat conduction due tothe filler, transitions to a sufficiently high value. The degree offiller required to reach this transition level may be too high and canoverpower the properties desired from the polymer binder such as lowcontact resistance. Another problem is that for some metal particles incontact with some polymer binders, the bare particle filler can poisonthe polymer cure such as by hindering or blocking the curing agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of an expanded view of a heat sink, a TIM 2,a heat spreader, a TIM 1, a circuit package, and a printed circuitboard.

FIG. 1B is an illustration of an expanded view of a heat sink, a TIM, acircuit package, and a printed circuit board.

FIG. 2 is an illustration of non-fusible particle filler material withina polymer matrix of a thermal interface material.

FIG. 3 is an illustration of an exploded view of thermal interfacematerials placed between computer components.

FIG. 4A is an illustration of one embodiment of a fusible particlefiller and a non-fusible particle filler in a phase change polymermatrix before reflow.

FIG. 4B is an illustration of the one embodiment of the fusible andnon-fusible filler in the phase change polymer matrix after reflow.

FIG. 5 is an illustration of one embodiment of a non-fusible mesh withina thermal interface material.

DETAILED DESCRIPTION OF THE INVENTION

A structure and method is disclosed for providing improved thermalconductivity of a thermal interface material (TIM) made of a phasechange polymer matrix and a fusible filler material. The TIM may alsohave non-fusible filler material and a percentage of non-phase changepolymer added to the phase change polymer matrix. The TIM, used to mateand conduct heat between two or more components, can be highly filledsystems in a polymeric matrix where the fillers are thermally moreconductive than the polymer matrix. In one application, the TIM, indispensable form or in sheet form, can be applied between a die orcircuit package, and a heat spreader, i.e. a TIM 1, and a second TIMapplied to the heat spreader between the heat spreader and the heatsink, i.e. a TIM 2. The heat spreader, which could be a composite or ametal such as Al (aluminum), AlN (aluminum nitride), or Cu (copper), isplaced between computer components such as the die and heat sink toconduct heat away from the die. In the following description, numerousspecific details are set forth such as specific materials, equipment,and processes in order to provide a thorough understanding of thepresent invention. In other instances, well known computer assemblytechniques and machinery have not been set forth in detail in order tominimize obscuring the present invention.

To remove more heat from the die during operation, thermal performancecan be increased by adding a phase change polymer, such as a silicone,combined with fusible fillers, such as solder powders, to form apolymer-solder hybrid TIM. During cure, the solder particles liquefy toconnect forming networks that are columnar structures with high thermalconductivity. This in turn enhances the flow of heat between the twointerfaces. This formation of a columnar structure between the twosurfaces in contact, i.e. die and heat sink for a TIM, die and heatspreader or heat sink for TIM 1, and heat spreader and heat sink for TIM2, improves the ability of the material to dissipate heat generated bythe die significantly.

The further addition of non-fusible fillers to create a TIM that has ablend of fusible and non-fusible fillers in the phase change polymermaterial strengthens the TIM mechanical properties and also can improvefiller uniformity within the TIM. The fundamental mechanism of heatconduction in mixed-filler phase change polymer matrix TIMs is primarilya combination of percolation between the non-fusible filler particlesand thermal conduction through solder bridges that connect thenon-fusible fillers, thereby lowering particle contact resistances. Thecontact resistances at the metal-TIM interfaces on the die and the heatsink are also lowered due to fusible solder wetting and adhering tothese interfaces.

At assembly, the phase change TIM can be placed between the matinghardware and a compressive force can be applied to the sandwichedcomponents. A reflow operation can then be accomplished that heats upthe component stack and can cause any fusible solder filler materialwithin the polymer binder to liquefy and flow. The times andtemperatures for the reflow operation can vary depending on the type andamount of filler and polymer binder used. The temperature range for areflow operation can be in the range of approximately 150-400° F. Onceliquid, the fusible solder can form columnar shapes that provide thermalconductive paths, which will increase the overall thermal conductivityof the TIM.

Phase change materials are generally olefinic polymer based and have thecharacteristics of changing from a solid to a liquid at predeterminedtemperatures. The polymer binder can have the phase change materialmixed with another polymer such as, for example, a thermoplastic. Thephase change material may not have good thermal conductivity in itsinitial state. When the temperature is increased by the heat from amating component or the ambient around it, the phase change materialwill change state to a liquid and flow into the cavities of the matingsurfaces such as on a heat sink, spreader plate, or device. This flow ofthe thermal interface material fills the cavities with the thermalinterface material and therefore provides a low thermal resistance. Thethermal interface material can change back to a solid when thetemperature is lowered, such as by removing power from the device.

There are many phase change materials with numerous additives to lowerthe thermal resistance. Thermal interface materials are applied indifferent thicknesses where the thickness variations can have more to dowith the flatness issue then the cavities. Some of the thermal interfacematerial types are designed to flow easier when they melt in order toprovide a thinner end result and therefore a lower thermal resistance. Acompromise is to allow easy flow and still keep it in place usingsurface tension. Relatively low application forces are required inmounting the device to these phase change materials where typical forcescould be in the 5 to 30 pound area.

FIG. 3 is an illustration of one embodiment of mixed-filler phase changepolymer matrix TIMs placed between computer components. A firstmixed-filler phase change polymer matrix TIM (TIM 1) 302 can thermallyconnect a circuit package 304 to a spreader plate 306 and a secondmixed-filler phase change polymer matrix TIM (TIM 2) 308 can thermallyconnect the spreader plate 306 to a heat sink such as a heatpipe 310.The formulation for the two TIMs may be the same or can be different inboth the type of phase change polymer matrix as well as the materialsand blend ratios for the fusible and non-fusible fillers.

FIG. 4A is an illustration of one embodiment of a thermal interfacematerial having both fusible filler and non-fusible filler in the phasechange polymer matrix. FIG. 4A represents the TIM 408 after theingredients have been mixed together but before a reflow operation hasoccurred. As mentioned, one advantage for using phase change material ina TIM 408 is to provide higher thermal performance. The higher thermalperformance is a result of the liquid phase, which can reduce thermallosses due to contact resistance while providing excellentconformability to the surfaces in contact. Phase change materials can beused as a binder in a TIM, TIM 1, or TIM 2, either as dispensable or asa sheet, and offering reworkability or non-reworkability thus providinga multitude of ways to use. The highly thermally conductive non-fusiblefiller particles 402 can be mixed with the less thermally conductivefusible filler 403 and then blended into the phase change polymer binder404 (least thermally conductive material in the TIM) where the mixturecreates single contact points 405 by the non-fusible particles 402.

FIG. 4B is an illustration of the fusible filler and the non-fusibleparticles in a polymer matrix after a reflow operation. During reflow,the fusible filler 403 wets the surface of the non-fusible particles402, coalesces and generally fuses the non-fusible particles 402together creating in the process larger cross-sections 406 of continuouspathways for heat conduction. A result of fusible filler 403 flow andthe increased cross-sectional areas 406 joining fusible particles 402,is that higher thermal conductivity can be created through the largerpathways than can be achieved via percolation by point contacts whenusing only non-fusible particle fillers.

In addition, any fusible filler contact areas with the mating computercomponents (die, spreader, etc, FIG. 3) can be wetted out providing goodthermal transfer across these contact area. When using fusible filler403 with non-fusible filler 402, the amount of total filler 402 and 403needed to obtain a value of thermal conductivity for the TIM 408 can belower than when using only non-fusible particle filler 402.

A partial list of fusible filler material 403 that can be added may bemetals and metal alloys such as In, InBi, InSn, BiSn, PbSn, SnAg,InPbAg, InAg, InSnBi, InGa, SnBiZn, SnInAg, SnAgCu, SnAgBi and InPb. Thefusible filler materials can be in the form of a powder. The fusiblefiller materials can be in the form of a solder having a low meltingtemperature and where there can be additives such as resins to aid inthe flow and wetting of the mating surfaces and to the non-fusibleparticles. A partial list of non-fusible particle filler material 403that can be added to the mixed-filler phase change polymer matrix arealuminum oxides, zinc oxide, aluminum, boron nitride, silver, graphite,carbon fibers, diamond, and metal coated fillers such as, for example,metal coated carbon fiber or metal coated diamond. The total weight offiller to total weight of mixed-filler phase change polymer matrix TIMcan be in the range of approximately 10-95% filler. The total weight offusible filler can be in the range of approximately 60-90% by weight ofthe total weight of the thermal interface material. The total weight ofnon-fusible filler can be in the range of approximately 5-50% by weightof the total weight of the thermal interface material. The fusiblefiller material can have a melting temperature approximately in therange of 100-250° C. The material choice for non-fusible material mayexclude lead, cadmium, mercury, antimony, and arsenic due tocontamination and safety hazard concerns.

Finally, when selecting the phase change material, forming a polymermatrix may be a blend of both phase change material and non-phase changematerial to tailor the properties desired in the polymer matrix.

FIG. 5 is an illustration of one embodiment of a mesh added to the TIM.The TIM 504 can use a phase change material 506 that is highlycross-linked, partially cross-linked, not cross-linked, or blendsthereof. During a reflow assembly operation, however, the lesscross-linked polymers with fusible filler 508 may compress with heat andassembly forces until there is contact between the two mating components(none shown). A hard stop may be placed within the TIM 504 to maintain aminimum bond gap between the components. The hard stop may be in theform of a mesh 502 made of non-fusible materials and that are highlythermally conductive. Mesh material that can be used includes aluminum,alumina, silver, aluminum nitride, silica coated aluminum nitride, boronnitride, carbon fiber, diamond and other metal coated inorganiccompounds. The mesh material 502 can be in large sheet form when addedto the TIM 504, also in sheet form. As a result, the mesh 502 can beapproximately a continuous piece when the TIM 504 is cut to shape foruse. Alternatively, the mesh material 502 may be smaller pieces of meshadded to the TIM 504 where in one embodiment, the mesh pieces can havean approximate shape that is 0.1″ square. As shown in FIG. 5, the mesh502 should lie flat within the TIM 504, i.e. flat with the length (L)and width (W) of the TIM 504 so as to limit a bond gap betweencomponents (not shown) to the thickness of the mesh 502.

The following is a description of one embodiment of a phase change TIM.The polymer matrix, including the phase change material, can be made upof polyolefins, epoxies, polyesters, acrylics, etc. comprisingapproximately 8% of the thermal interface material by weight. In oneembodiment, the phase change polymer is a liquid above 45° C. The soldermaterial can be indium comprising 77% of the thermal interface materialby weight. Indium has a melting temperature of 157° C. and does notattack phase change resin when melted at a temperature above 157° C. Thenon-fusible particles can be aluminum comprising 15% of the thermalinterface material by weight. The solder particles and the non-fusibleparticles thus comprise approximately 92% of the thermal interfacematerial by weight Aluminum has a melting temperature of approximately1200° C., the filler particles thus melt at a temperature which is 1043°C. higher than a melting temperature of the solder particles.

The composition is heated from room temperature of approximately 30° C.to approximately 170° C., which is above the melting temperature ofindium so that the indium solder particles melt. The composition ismaintained at 170° C. for approximately two minutes, i.e. untilsufficient agglomeration has occurred. The composition is then cooled toa temperature of approximately 125° C., which is below the soldermaterial's melting point and the solder particles solidify. Curing timeand temperature may be varied and are related to one another.

Heat is generated by the die and transferred through the fillerparticles to the thermally conductive member (integrated heat spreaderor heat sink). Differences in thermal expansion of the die and thethermally conductive member cause stresses on the material that areprimarily absorbed by the phase change matrix material. The resistanceto heat flow is characterized by a term, Rjc, which indicates thethermal resistance between the die junction and the top surface of theconductive member.

Such TIM material can be applied via various assembly methods. With suchphase change TIM material pre applied (screen printing, perform, etc.)to the thermal conductive member (such as the heat spreader or heatsink), package assembly builds showed an average Rjc of 0.17-0.18° C.cm²/W. With the phase change TIM material dispensed, package assemblybuilds showed an average Rjc of 0.18-0.19° C. cm²/W. The phase changeTIM material placed between the die surface and only a copper plate asthe heat sink, showed package assembly builds with an average Rjc of0.22-0.23° C. cm²/W.

The present invention takes advantage of fusible material flow toimprove thermal conductivity that would otherwise occur by percolation(non-fusible particle-particle contact only). This advantage is gainedby producing larger continuous thermal pathways adding to the point-topoint non-fusible particle contact in a phase change polymer matrix. Thefusible and non-fusible filler mix can produce a TIM having higheroverall heat conductivity than a non-fusible particle filled TIM for agiven amount of filler by percent weight.

1. An electronic assembly comprising: a microelectronic die; a heatspreader; a heat sink; and first and second thermal interface materials,the first thermal interface material being between the microelectronicdie and the heat spreader and the second thermal interface materialbeing between the heat spreader and the heat sink, the first and secondthermal interface materials comprising a first and second phase changepolymer, respectively, each phase change polymer selected from the groupconsisting of a polyolefin, an epoxy, a polyester and an acrylic, andthe first phase change polymer different from the second phase changepolymer, a solder material, said solder material different than saidphase change polymer, and a plurality of thermally conductivenon-fusible particles, the solder material interconnecting thenon-fusible particles to form a plurality of columnar structures withineach of the first and second phase change polymers.
 2. The electronicassembly of claim 1, wherein at least one of the first and secondthermal interface materials further comprise a non-phase change polymer.3. The electronic assembly of claim 1, wherein at least one of the firstand second phase change polymers is a liquid above 45° C.
 4. Theelectronic assembly of claim 1, wherein a non-fusible mesh is placedwithin at least one of the first and second phase change polymers. 5.The electronic assembly of claim 1, wherein the non-fusible particlesinclude at least one of glass fiber, graphite fibers, carbon fibers,boron nitride, aluminum oxides, zinc oxide, aluminum, silver, graphite,carbon fibers, diamond, metal coated carbon fiber, and metal coateddiamond.
 6. The electronic assembly of claim 5, wherein the non-fusibleparticles are excluded from the group consisting of lead, cadmium,mercury, antimony and arsenic.
 7. A method of constructing an electronicassembly comprising: interconnecting a microelectronic die and a heatsink with a first thermal interface material, the first thermalinterface material comprising a first phase change polymer selected fromthe group consisting of a polyolefin, an epoxy, a polyester and anacrylic, a solder material different from said first phase changematerial, and a plurality of thermally conductive non-fusible particles,the solder material interconnecting the non-fusible particles to form aplurality of columnar structures within the first phase change polymer;and interconnecting a heat spreader and the heat sink with a secondthermal interface material, the second thermal interface materialcomprising a second phase change polymer selected from the groupconsisting of a polyolefin, an epoxy, a polyester and an acrylic anddifferent from the first phase change polymer, a solder materialdifferent from said second phase change material, and a plurality ofthermally conductive non-fusible particles, the solder materialinterconnecting the non-fusible particles to form a plurality ofcolumnar structures within the second phase change polymer.
 8. Themethod of claim 7, wherein at least one of the first and second thermalinterface materials further comprises a non-phase change polymer.
 9. Themethod of claim 7, wherein at least one of the first and second phasechange polymers is a liquid above 45° C.
 10. The method of claim 7,further comprising placing a non-fusible mesh within at least one of thefirst and second phase change polymers.
 11. The method of claim 7,wherein the non-fusible particles include at least one of glass fiber,graphite fibers, carbon fibers, boron nitride, aluminum oxides, zincoxide, aluminum, silver, graphite, carbon fibers, diamond, metal coatedcarbon fiber, and metal coated diamond.
 12. The method of claim 11,wherein the non-fusible particles are excluded from the group consistingof lead, cadmium, mercury, antimony and arsenic.